I'll do it when I'm worried about fatigue. I don't even look for a speed guy, typically #9 is a good fielder with low PA and I want to keep his glove in the field. Of course that's a different reason than the LaRussa strategy, which I don't agree with. Tony first did it simply to pad McGwire's stats by getting him more baserunners, then when challenged he claimed it made strategic sense.
If my pitcher is a half-decent hitter then I don't mind batting him 8th.